genes

• Base sequence

  • DNA is made from four different bases (A, T, C, G).
  • A gene is a sequence of bases in DNA.
  • Each gene has a specific base sequence.
• Locus

  • The specific location of a gene in the DNA is called the locus.
  • The locus is a fixed position.
• Codons

  • Three bases in a gene are called a triplet or a codon.
  • A specific codon encodes a specific amino acid.
  • The codons present in a gene and the sequence of the codons are translated into a sequence of amino acids.
  • Together these amino acids form a polypeptide.
• mRNA

  • The sequence of bases in a gene is made into a polypeptide by copying DNA into a molecule called messenger RNA (mRNA).
  • mRNA is then copied into the corresponding polypeptide.
• Functional RNA molecules

  • Some genes encode a functional RNA molecule instead of a polypeptide.
  • Functional RNA molecules are necessary for protein synthesis.
  • E.g. Transfer RNA (tRNA) helps find the correct amino acids for a specific codon.
  • E.g. Ribosomal RNA (rRNA) are a part of ribosomes, the centre of protein synthesis.
• A large proportion of the genes in eukaryotic DNA do NOT code for any polypeptides. These genes have no biological function and are referred to as non-coding genes.
• Exons

  • The coding regions in a gene are called exons.
  • Sequences of coding genes are interrupted at regular intervals by non-coding regions.
• Introns

  • The non-coding regions in a gene are called introns.
  • When a gene is translated for protein synthesis, the introns are removed from the sequence.
• Multiple repeats

  • There are also regions of non-coding multiple repeats in eukaryotic DNA.
  • Multiple repeats are random sequences in the DNA that will be repeated many times (e.g. CGCCGCCGCCGC).
• The genetic code is the combination of triplet codes in the genome. There are three important features of the genetic code:

  • non- overlapping
  • universal
  • degenerate
• features of the genetic code:

  Non-overlapping

  • The genetic code is non-overlapping.
  • The non-overlapping nature of DNA means that there is no overlap between triplet codes.
  • Each triplet code is separate from the other triplets in the chromosome.
• features of the genetic code:

  Universal

  • The genetic code is universal.
  • The same four bases (A, T, G, C) are used in the DNA of every organism.
  • The same codons encode the same amino acids in every organism.
  • Codons in DNA are transcribed into mRNA and translated into amino acids in every organism.
• features of the genetic code:

  Degenerate

  • The genetic code is degenerate.
  • Some amino acids are encoded by more than one codon (e.g. tyrosine is encoded by TAT and TAC).
  • There are 64 possible triplet codons and only 20 amino acid